In the production of pig iron in a pig iron production unit, such as furnace or a melt reduction unit for example, such as a melter gasifier used in the COREX® or FINEX® method, a reduction gas is obtained by gasification of carbon carriers by blowing in hot air or an oxygen stream. Oxidic iron carriers are reduced by means of this reduction gas and subsequently the reduced material obtained is melted into pig iron.
In the melter gasifiers used in the COREX® and FINEX® methods oxygen nozzles are built into the circumference of the melter gasifier between hearth and char bed of the melter gasifier, in order to blow in the oxygen for the gasification of carbon to produce the reduction gas and provide the energy necessary for smelting the iron carriers as evenly as possible at the circumference of the melter gasifier into the bed of the melter gasifier. When the iron carriers are smelted liquid pig iron and liquid slag are produced. The area of the melter gasifier below the oxygen nozzles, in which there is no throughflow by reduction gas, is referred to as the hearth in such cases. Located in the hearth are liquid pig iron, liquid slag and a part of the char. Thermally degased carbon carriers are referred to as char. The area of the melter gasifier lying above the oxygen nozzle is referred to in such cases as the char bed; as well as liquid pig iron and liquid slag and char, it also contains unmelted and partly reduced iron carriers and additives. Reduction gas which is formed by converting the introduced oxygen flows through the char bed. The oxygen streams entering the melter gasifier through the oxygen nozzles form what is known as the raceway within the melter gasifier, in which gasification of carbon carriers is already taking place, wherein reduction gas is already being produced. Raceway in such cases is to be understood as the eddy zone in front of the oxygen nozzles, in which the reduction gas is produced from oxygen and carbon carriers. The term eddy zone in this case reflects the highly turbulent eddy layer-like flow conditions in the area of the raceway. The incoming oxygen stream creates a cavern in the material of the char bed. The cavern is produced by the impulse of the arriving oxygen stream and by the gasification reaction of the oxygen with the char. The area of the cavern is referred to as the raceway. By comparison with the char bed, which represents a liquid bed, the raceway has a much higher number of gaps. The raceway extends in accordance with the arrangement of the oxygen nozzles on the circumference of the melter gasifier inside the melter gasifier in a horizontal plane. The cross-sectional surface which is formed when viewed from above by the length of the raceway, is also referred to as the active ring surface wherein, in the term active ring surface, the word active refers to the fact that drainage of liquid pig iron and liquid slag is carried out especially well by the raceway because of the number of gaps of the raceway, and the reduction as produced by gasification of carbon carriers enters from the raceway into the char bed. The width of the active ring surface is determined by the longitudinal extent of the raceway and thus by the penetration depth of the oxygen stream.
Even with a furnace in which hot blast or oxygen is blown in through nozzles, also called blast form raceways, distributed accordingly around the circumference of the furnace, raceways with active ring surface form in the area of the nozzles.
For the char bed of a melter gasifier, with the usual use of an oxygen stream of technically pure oxygen with a temperature of between −15° C. and +45° C. and because of the smaller diameter of the oxygen nozzles used by comparison with the packed bed present in a furnace operated with hot blast, a far lower penetration depth of the oxygen stream into the bed material is produced. Thus, through the shorter or respectively narrower raceway in the char bed, a comparatively small active ring surface at the circumference of the melter gasifier is produced by comparison with a furnace operated with hot blast, through which the gas permeability for reduction gas into the char bed or the drainage of liquid pig iron and liquid slag into the hearth respectively are comparatively worse. Furthermore, by comparison with furnaces operated with coke, by the use of lump coal and/or coal briquettes as carbon carriers, the hydraulic diameter of the char matrix in a melter gasifier is reduced, whereby the flowing away of liquid pig iron and/or liquid, specifically of highly viscous slag, is rendered more difficult, which can lead to problems from a buildup of liquid pig iron and/or liquid slag in front of the oxygen nozzles.
An increase in the penetration depth of the oxygen stream into the bed, both in a furnace operated with oxygen and also in a melter gasifier, would greatly increase the active surface and thus improve the drainage of liquid pig iron and of liquid slag.
The reduction gas essentially flows upwards. Viewed in the direction of flow of the reduction gas, after the raceway, i.e. above the raceway, undesired liquidized areas are produced in the bed of the melter gasifier or furnace, also called bubble or channel formation. Into these areas a quantity of gas at a high pressure enters the bed of solids and the mixture of solids and gas produced behaves like a liquid. The formation of liquidized areas is unwanted, because it can lead to blowing through the bed of the melter gasifier or of the furnace. Blow-throughs lead to suddenly increasing changes of the gas flow, dust loading and combination of the gas conveyed out of the melter gasifier or furnace, which makes it more difficult to manage the operation of such units. Furthermore with blow-through these particles are expelled from the melter gasifier or furnace into lines for drainage of reduction gas or blast furnace gas.
Liquidized areas are also unwanted since an optimum phase conduction of gas and solids is prevented by them. In liquidized areas the result can be a mixing of material from the upper and from the lower area of the char bed—thus for example iron oxide reaches into the lower area of the char bed from the upper area of the char bed and completely reduced and partly already melted iron from the lower area of the char bed will be transported into its upper area.
On introduction of a larger quantity of gas, specifically a larger quantity of oxygen into the bed, with melter gasifier and furnaces driven by oxygen, the danger of liquidized areas arising increases while the penetration depth remains the same.
If the penetration depth of the oxygen stream is increased in relation to a basic state, a specific quantity of gas can escape from the raceway into the bed via an increased surface compared to the basic state. Accordingly pressure conditions leading to formation of liquidized areas in the vicinity of the oxygen nozzles compared to the basic state will occur less often spatially and temporally, and as a result liquidized areas will be less large and occur less frequently in the vicinity of the oxygen nozzles.
In a melter gasifier, in the area of the entry of the oxygen stream into the bed, i.e. the raceway, because of the high speed of flow—which is higher by a multiple compared to a furnace, the chemical and thermal volume expansion, and because of the smaller char size by comparison with the average size of the coke in the furnace, an eddy zone occurs. In accordance with known conventions practically no increase of the penetration depth is achieved by a higher flow speed of the oxygen stream. An increase in the flow speed of the oxygen stream would increase the mechanical stress on the char. The mechanical stress would increase by impulse transmission between the particles of the oxygen stream and the components of the char bed—i.e. of the char—and consequently by impulse transmission between the components of the char bed themselves. Through the friction or decay of the char respectively caused by the impulse transmission, or by the mechanical stress resulting from said transmission, more fine grain would be formed in the eddy zone.
For the decay of the char the specific impulse transmitted per unit of space is the defining variable. The characteristic variable for this is the impulse force, which represents the specific impulse related to a unit of surface.
More fine grain in the eddy zone however leads to a reduction of the hydraulic diameter of the eddy zone of the raceway, which in its turn adversely affects the drainage of liquid pig iron and of liquid slag through the active ring surface.
In the case of a packed bed in a furnace, an increase in the penetration depth can be achieved by increasing the oxygen speed.
In this case a significant difference arises between a furnace operated with hot blast and a furnace operated with oxygen. The penetration depth of the oxygen stream is far less in a furnace operated with oxygen compared to the penetration depth of hot blast in a furnace operated by hot blast of the same power. The reason for this is because the mass flow of introduced gas is lower with an oxygen stream, since a large amount of nitrogen is not brought in with the required amount of oxygen as it is with hot blast. In the case of a furnace operated with oxygen, to achieve the penetration depth which is present in a furnace of the same output operated with hot blast, the oxygen speed would have to be increased by comparison with the speed of the hot blast—however this would result, as previously described, in increased mechanical destruction of the coke in the furnace as a result of impulse transmission and accordingly through fine grain formation to a lower gas permeability of the packed bed in the furnace.